The invention is directed to an actuator and in particular to a magnet actuator for driving an objective lens movable in two dimensions. The actuator is supported about a shaft being movable rotationally about the shaft and movable rectilinearly in the axial direction of the shaft.
Two dimensional actuators drive the objective lens of an optical head in, for example, an optical disk memory drive, in a first rotational direction (.theta.) and a second rectilinear direction (Z) in the axial direction of the shaft. These two dimensional actuators are known as .theta. - Z actuators. The majority of prior art .theta. - Z actuators are moving coil actuators such as is shown in Japanese Laid-Open Unexamined Patent Application No. 210456/82. A second type of two dimensional actuator is a moving magnet objective lens actuator such as is shown in Japanese Laid-Open Unexamined Patent Application No. 37830/88.
The moving coil actuator has been satisfactory. However, it suffers from the disadvantages that the feeder line disconnects from the moving coil, the bond deteriorates due to overheating of the coil and the coil changes due to thermal deformation resulting from the overheating of the coil.
Additionally, the process for connecting the feeder line to the moving coil is intricate and troublesome. The feeder line also exerts adverse influences during high speed operation of the unit. Furthermore, non-uniform configurations of the coil cause an imbalance in the movable unit's mass causing a higher order resonance preventing high speed operation. Accordingly, when the moving coil actuator is applied to an optical head in an optical disk memory device the optical disk rotation frequency cannot be increased, limiting the data transfer rate. Modification of the coil's specifications i.e. the number of lines, line diameter and the like, will lead to variations in mass of the movable unit. Accordingly, trial and error processing requiring modification in the design of the actuator must be performed to obtain in optimal coil specifications.
The moving magnet actuator has also bee satisfactory However, it suffers from the shortcoming that a plurality of magnet circuits and magnets are necessary to obtain the desired two dimensional operations. This type of construction results in a complicated structure. Accordingly the mass of the movable unit often becomes unbalanced. Furthermore, an increase in the movable unit's weight adversely affects the high speed response of the actuator. Additionally, due to the complex structure, production of a moving magnet actuator is expensive.
Accordingly, it is desirable to provide an actuator suitable for use as an objective lens of an optical head capable of operation in two dimensions which overcomes the shortcoming of the prior art devices described above by using a cylindrical magnet having multi-polar magnetization in a radial direction, having magnetic boundaries in both a circumferential direction and axial direction and a yoke having magnetic poles each facing the magnetic boundaries.